1. Field of the Invention
The invention relates to thermoplastic molding compounds which have high impact strength when cold.
2. Discussion of the Background
There are stringent practical requirements which are placed on the physical properties of thermoplastic molding compounds, e.g. molding compounds to be used for manufacturing piping and the like. The materials must be inert to the goods being conveyed, and must be dimensionally stabile when thermally stressed. For piping to be used outdoors, the materials must be weather resistant, i.e. they must materially preserve their useful properties when acted upon by solar radiation, airborne moisture and humidity, cold, and the like.
Generally, all of the familiar polyamides (PA) are suitable for molded parts. However, there are limitations, particularly for outdoor use. Thus, there are no known industrially used polyamides which have at least one monomer with .ltoreq.10 carbon atoms in the hydrocarbon chain which are not appreciably sensitive to water or airborne moisture. Attack by water causes these polymers to substantially lose their impact strength, particularly impact strength at low temperature ("Kunststofftaschenbuch" 19th Ed., pub. C. Hanser Verlag, pp. 294-295, table).
Polyamides with more carbon atoms in the hydrocarbon chain are more suitable, e.g. polyamides 11 and polyamides 12, because they have better weather resistance and thereby better cold impact strength. However, these industrially readily available higher polyamides have the disadvantage that when plasticizers are employed with them they lose their impact strength. This problem must be alleviated by using an impact strength modifier (Ger. AS 27 16 004). This compels one to work with a 3-component system, which is a drawback.
A further disadvantage of polyamides 11 and polyamides 12 is that they are produced from a lactam or the corresponding aminocarboxylic acid, and as a matter of course the raw polymer contains monomers and oligomers which must be removed from the product in an additional process step.
Plasticizers and their use in polyamides are per se known.
A continuing need exists for a molding compound based on a polyamide, which does not lose its cold impact strength even when plasticizers are employed, or under the action of weathering (e.g. moisture). Preferably the material should comprise a 2-component system.